Electron beam source including a pilot nonthermionic, electron source

ABSTRACT

A device for generating an electron beam including a pilot gun which generates a pilot electron beam by nonthermionic means. The pilot electron beam is used to bombard a main cathode from which a main electron beam is generated.

United States Patent 1 3,610,986

[72] Inventor James R. King [56] References Cited {21] APPL ggfgg UNITEDSTATES PATENTS [22] Filed May 1, 1970 3,393,339 7/1968 Hill et al.250/419 SB X [45] Patented O t, 5, 1971 3,482,096 12/1969 Lewis et al.313/74 X [73] Assignee Union Ca bid Corporation 3,517,240 6/1970Dickinson 313/63 New York, N.Y. Primary ExaminerRoy Lake AssistantExaminer-Palmer C. Demeo Atto e Paul A. Rose, Harrie M. Humphreys andDominic s4 ELECTRON BEAM SOURCE INCLUDING A PILOT M W NONTHERMIONIC,ELECTRON SOURCE J. Termmeuo 7 Claims, 7 Drawing Figs.

[52] US. Cl 313/63,

313/74, 313/231, 313/318 ABSTRACT: A device for generating an electronbeam in- [51] Int. Cl H0lj 5/50, eluding a pilot gun which generates apilot electron beam by H01 j 17/26 nonthermionic means. The pilotelectron beam is used to [50] Field of Search 313/63, 74, bombard a maincathode from which a main electron beam is 230, 231, 318; 250/419 SBgenerated.

PATENTEU um 5197] 1 3510.99

' sum 1 0F 5 H.V. '10 KV INVENTOR. JAMES R. KING ATTORNEY PATENTEU um 5mCATHODE VOLTAGE SHEET 2 [IF 5 POTENTIAL DISTRIBUTION (PARALLELELECTRODES) 7 move I oaop CATHODE DROP Lu 0 O 2 2 b U m g g g F a. 2 B 5:5 3 E LIGHT DISTRIBUTION 0 :1 Lu Q X m Q O x l I! z O a: g o E g 3 s wz 2 z o g a POSITIVE COLUMN 2 E I E 1 m {E 42 U u INVI'JNIHH. JAMES R.KING N N LUNINUL o RING VOLTAGE 8 CURRENT m PATENTEB 0m 5 |97| SHEET 5OF 5 CONTROL RING VOLTAGE CURRENT 5KV ACCELERATING VOLTAGE 3" CATHODE TOTARGET DISTANCE INVFINIUR. JAMES R. KING ELECTRON BEAM SOURCE INCLUDINGA PILOT NONTHERMIONIC ELECTRON SOURCE This invention relates to animproved electron beam gun and more particularly to such a gun whicheliminates electromagnetic fields around the main cathode of the gun andvastly increases the operating life of the heated emitter of the maingun.

Apparatus for generating electron beams by utilizing a heat source whicheffects emission of electrons from a cathode by thermionic emission arewell known. Also well known are the gaseous type electron generatingguns frequently referred to as the plasma or cold cathode-type gun. Itis also generally acknowledged that the power density of a hot cathodegun is greater than that achieved by a cold cathode gun. Up until now,thermionic emission was achieved in several ways, the most common beingresistance heating of the filament. This, of course, depends upon bodyresistance of the filament material to the passage of high electricalcurrents.

Resistance heating has several accumulative drawbacks, not the least ofwhich is related to the fact that emission is an area dependentfunction, and therefore, the greater the emission required from the gun,the larger the emitting area must be. The larger the area, the lower theresistance of the filament as resistance is inversely proportional tothe cross sectional area of the conductor. As a consequence of H=FR theheating current must be greater to obtain greater emission which in turncreates a greater magnetic field around the filament, and of course,requires cables of larger cross section to carry the required current.

The first of the problems is to maintain a uniform heating currentthrough the cathode which is changing resistance with temperature.Further, there is a decided change in contact resistance at the pointwhere the cathode is clamped to the electrical heating supply. Theseclamps must function both mechanically and electrically to hold thecathode mechanically firm while it is being heated from ambienttemperature to approximately 2300" K and to hold the cathode withunvarying electrical contact to conduct the required high currentthroughout the temperature range and while held for prolonged periods athigh temperature.

A resistance-heated cathode has a very sensitive geometry, ordistribution of resistance, such that it is difficult to restrictemission to the desired area. The cathode must be designed so that thearea of highest resistance, and, therefore, the greatest heat, isconcentrated at the center of the filament. Under ideal conditions thisarea, and only this area, is brought to emission temperature. lnaddition to the geometry of the cathode itself, the attachments to thefilament must be designed to function as heat sinks which will preventextraneous heating.

The magnetic field surrounding the filament, as a result of the heatingcurrent through the filament, enters the interelectrode space betweenthe anode and cathode .of the electron beam gun and causes an unwanteddiversion of electrons and thereby, a bending of the electron beam. Itcan be shown that the initial angle of deflection exerted by thefilament field depends upon the heating current which is varied for manyreasons to satisfy the emission requirements. Further, the initial angleof deflection depends upon the velocity of the electrons as they enterthe interelectrode space as determined by the square root of theaccelerating voltage, thus creating a complex interdependency.

A second method of heating the emitter for the welding gun is lesscommon due to technical difficulties involved in the solution, butnevertheless, exists in practice. This system is often called anindirectly heated or bombarded cathode, and it consists of a smallelectron beam gun behind the main gun. The sole purpose of the first gunis to produce an electron beam that is accelerated at low DC voltage tobombard the emitter of the main gun.-ln this case, advantage is taken ofthe kinetic energy conversion to heat energy at the point of impact ofthe accelerated'electrons, e=%mv*. The bombarding gun consists of aresistance-heated filament which is placed at some negative potential,say 1000 volts negative. The emitter of the welding gun is the targetfor the bombarding gun and is made positive with respect to the filamentof the bombarding gun. Basically, the emitter is serving as the targetanode of a crude work focus gun. It should be noted that approximatelythe same power is required to heat the given mass of the emitter,regardless of the method of heating used. In this case, however, lowercurrents are used in the milliampere range in conjunction with a highervoltage, than is used for resistance heating.

This technique is extremely effective and produces a very narrow andstraight beam due to the absence of a magnetic field around the emitterof the main gun. The other relative disadvantages of the magneticallyinduced mechanical forces are also known eliminated. The system,however, has the in herent weakness that the bombarding gun is makinguse of a resistance-heated filament which will also eventually fail dueto the geometry, thinness, and magnetically induced mechanical stresses.

Accordingly, it is the main object of this invention to provide a novelelectron beam generating gun which eliminates unwanted electromagneticdeflection of the beam, decreases thermal aberration, and has vastlyimproved emitter life.

Another object is to provide a gun that can be used with electricalsupply cables which are small in size and are sufficiently flexible topermit convenient coiling thereof in the gun chamber.

A further object is to provide a gun wherein the main cathodetemperature is varied in response to changes in the main beam current inorder to maintain constant current or to vary current according to apredetermined program such as a slope or taper program.

Still another object is to provide a gun provided with means forcontrolling backfiring through the gas due to the negative potential ofthe main cathode with respect to the chamber walls.

These and other objects will either be pointed out or become apparentfrom the following description and drawings wherein;

FIG. 1 is a front elevation view partially in cross section of anembodiment of the invention.

FIG. 2 is a graphic comparison of voltage distribution and lightdistribution in a normal glow discharge.

FIG. 3 is a sketch showing the important regions of the glow dischargein relation to the cold cathode gun.

FIG. 4 is a front elevation view partially in cross section of a secondembodiment of the invention.

FIG. 5 is a front elevation view partially in cross section of a thirdembodiment of the invention.

FIG. 6 is a graph of a typical curve showing variation of beam currentand control ring voltage as a function of pressure.

Briefly stated, the objects of the invention are accomplished by anelectron beam gun which includes a pilot gun comprising an enclosureprovided with a relatively low-pressure ionizable gaseous medium whichserves as the source of electrons for the gun. Positioned in one wall ofthe enclosure is the pilot cathode. The pilot cathode has a concavesurface with a radius of curvature approximately twice the cathoderadius to give a convergent beam of electrons having a focal pointbeyond the Faraday dark space. Positioned in the same wall as the pilotcathode concentric with and spaced from the pilot cathode is aring-shaped pilot anode. An accelerating voltage for the electrons isapplied between the pilot cathode and pilot anode.

A main cathode emitter is positioned in the wall of the enclosureopposite the pilot cathode and is maintained at the same potential asthe pilot anode. The distance of the main cathode-emitter from the pilotcathode is always beyond the Faraday dark space when operated within thedesired pressure range-The main cathode-emitter is bombarded on itsupper the emitter 360 but exposes its top surface to the bombarding beamand exposes a circular disc of a diameter appropriate to the desiredemission area on its lower surface. The structure surrounding theemitter on the top side is a part of its electrical circuit of the coldcathode pilot gun. The structure in the vicinity of the emitter may beflat, concave, or convex. The convex curvature is preferred as it mayaid in forming the equipotential surfaces between the pilot cathode andthe emitter.

The lower side of the structure is electrically a part of the main gunand its shape primarily determines the shape of the equipotentialsurfaces between the main cathode electrode and the anode electrode. Itis taught by the science of electron optics that the geometry of theequipotential surfaces between the cathode electrode and the anodeelectrode determines the electrostatic lens effect on the passage ofelectrons.

It can be seen that the upper and lower parts of the structuresupporting the cathode emitter function simultaneously in two differentcircuits. The upper portion functions as a part of the anode electrodeof the pilot gun and the lower portion functions as the cathodeelectrode of the main gun.

Spaced from the main cathode electrode is a main anode. Impressedbetween the main cathode electrode and main anode is an acceleratingvoltage to accelerate the electrons emitted from the main cathodeemitter toward a workpiece. The accelerating voltage between the maincathode and anode is usually high relative to the pilot acceleratingvoltage but is not dependent upon any particular ratio or relationship.

The pilot gun of this invention is a plasma or cold cathode guncontaining a low-pressure ionizable gas usually maintained at a pressureof from about to 30 microns, although the pilot gun can be operated athigher or lower pressures depending on the gas, voltage and electrodespacing. The gases can be ambient gases or inert gases, although inertgases are preferred for protection of the main cathode emitter. Thepilot gun can be operated at various voltages and power levels; however,in this invention the pilot gun is preferably operated at a voltage upto about kilovolts (kv.) and a maximum current of 50 ma.

The main gun is usually operated at a pressure of approximately 0.]micron or less, at a voltage of about 60 kilovolts (kv.) and a currentof 500 ma. which is wholly dependent upon the area and temperature ofthe emitter to produce an electron beam of 30 kilowatts. The work isusually placed in a chamber maintained at a pressure in the area ofabout 5 microns, although the pressure range might be at any value frominfinitely low to l or above 1 atmosphere when the gun is used for highvacuum, intermediate vacuum, or in atmosphere. However, utilizing theconcept of the invention, electron beams of different powers can beobtained by changing pressures, voltages, electrode spacing and kinds ofelectrodes and gases within the pilot gun such that emitters of greateror lesser area are heated to the appropriate emitting temperature.

Referring now to the drawing in FIG. 1, the electron beam gun is showngenerally at G. The pilot gun portion of gun G is shown at P. Such pilotgun has a pilot cathode member 1 positioned in one wall 3 of aninsulator enclosure 5 of pilot gun P. Outside of the pilot cathode l andinsulated therefrom by insulator material 2 is a pilot anode 7 having aflange plate 9 at its top portion. Outside the pilot anode 7 is acontrol ring 11. Positioned in the wall 13 opposite the pilot cathode lis the main cathode emitter l5. Main cathode 16 is electricallyconnected to pilot anode 7 by conductor 17. The main function ofconductor 17 is to carry electrons back from cathode emitter throughpilot anode 7 to pilot cathode l. The main cathode emitter 15 ispreferably a tungsten disc, although any good electron emitting materialmay be used such as tantalum or coatings having a low work function.Supporting main cathode emitter 15 is main cathode electrode 16. Spacedfrom said main cathode l6 and downstream therefrom is the main anode l9.Downstream from the main anode 19 is a main beam focusing coil 21.

The pilot cathode may be made of any suitable pure, alloyed or coatedmaterial such as molybdenum, copper, steel, and etc., although aluminumis preferred. it is well known that plasma cathode guns, having a flatcathode orifice plate produce a divergent electron beam. The powerdensity of such beams is usually low so that some focusing means must beemployed to form a convergent beam having higher power density which canbe used as a heat source. In this invention the pilot cathode has ashallow concavity which serves as the plasma cavity in which the plasmathat is the source of electrons is generated. This cavity has thecharacteristics of an electron lens. With this type of lens, whenelectrons approach the lens from the side of lower field strength, inthis case the plasma cavity in the pilot cathode, the lens action isconvergent. This convergence is obtained by the concave curvature of thecavity which results in equipotcntial surfaces which are convex and,therefore, exert a convergent action on the electrons.

It was discovered that a successful curvature of the pilot cathodeapproximates a shallow curve having a true radius, although Pierceshapes or parabolic shapes will function to a lesser degree. it wasfound that the radius of the curve is related to the CD. of the cathodesuch that the radius is approximately equal to twice the cathode radius.The choice of radius determines the depth of the curve; however, curvesof smaller radii will function, provided that the depth does not exceed0.130 to 0.180 inches. However, the most stable operation is achievedwith larger radii.

The vertical position of the focal point was inversely related, to thedepth of curvature; i.e., the deeper curvature, the nearer the focalpoint to the face of the cathode. On one test unit the focal point of acurvature of 0.750 radius and a depth of 0.250 was actually up insidethe cavity, whereas a depth of 0.080" projected the focal point severalinches from the cathode, and a depth of 0.130 projected the focal pointapproximately 2.5 inches from the cathode. As the focal point isprojected farther away, the diameter of the focal region is alsoincreased, thereby reducing the power density.

The plasma cathode gun involves the phenomenon of normal glow discharge.A normal glow discharge is divided up into areas of light and darkspaces of varying intensity. The bright areas are either areas of highionization or areas of recombination of electrons and positive ions. Thedark areas are conversely those of low ionization and recombination. Itis well known that the space nearest the cathode is called the Astondark space. The cathode glow space is the area in which the electronshave reached the critical minimum velocity for the given gas pressure.Next is the cathode dark space. In the cathode dark space, the electronsare above critical velocity and leave behind a large quantity ofpositive ions that were originated in the cathode glow space. Thenegative glow region is one of maximum potential drop and is extremelybright (the brightest of the entire column) and is an area of maximumionization and recombination due to the presence of large numbers ofions, electrons, and gas molecules. The dark space beyond the negativeglow region is the Faraday dark space and is one of potential minimum,with respect to cathode potential, due to the large number of electronsleaving the negative glow region. The electrons are slowed belowcritical ionizing velocity in this region and negative chargespredominate. The so-called positive column is a plasma of equal chargesof negative and positive ions. Following these are the anode glow andanode dark spaces which are of little consequence.

A graphic comparison of voltage distribution and light distributionbetween parallel electrodes is shown in FIG. 2. The main regions ofdischarge in relationship to the pilot gun elements are as shown in FIG.3, wherein smaller parts bear similar reference characters.

It was found that a work surface (ground or positive potential) may bepositioned at any point along the vertical height of the positive columnwith little change in beam current as long as it does not come intophysical contact with the Faraday dark space 35 (see FIG. 3). Uponcontacting the dark space with the work surface, the beam is suddenlyreduced in intensity from its maximum value to some extremely low valuewhich may be only one-tenth of the maximum. If the work is furtherraised until it contacts the now faint negative glow region 34, the beamcurrent will decrease even more, or will extinguish entirely. Due to thedrastic change as these areas are approached, we say that the dischargequenches when contact with the Faraday dark space is made.

Accordingly, while it is desirable from a beam power intensity point ofview to have a focal point as close as possible to the cathode to form asmall focal circle having a high-power density, it is not possible toutilize such a short focal distance due to the position of the Faradaydark space.

The distance of the Faraday dark space from the face of the cathode isdetermined primarily by the operating pressure, and secondarily, by thegun design and the applied voltage. Therefore, for a given set of designparameters the dark space must be tolerated at whatever distance it ispositioned. It is known, as discussed hereinabove, that any surface thatpenetrates the dark space will diminish (quench) the beam current, andthus it is imperative that the focal point distance from the pilotcathode be chosen to be somewhat greater than the pilot cathode to thelower edge of the Faraday dark space distance.

ln order to accomplish a focal distance greater than the dark spacedistance, some power density must be sacrificed due to a larger beamcross section at the longer distance.

ln operation, in the embodiment shown in FIG. 1, the pressure in thepilot gun P is maintained between and 20 microns and is controlled byflowing gas from a source through an inlet 23 into a bubbler unit 25filled with high dielectric fluid compatible with a vacuum environment.The purpose of the bubbler will be described hereinafter. From thebubbler unit 25 the gas flows into chamber C formed by enclosure 5. Thetop of the pilot gun P has a hole or holes 27 which communicatesdirectly with the vacuum chamber VC so that the pilot gun chamber C iscontinually pumped by the vacuum system connected to the chamber VC atport 29. The gas might also be ducted out of the gun by a tube andindependent pumping system. The pressure in the chamber C is maintainedby the flowing gas e.g. 100 cc./min. and such flow is small enough notto affect the actual pressure of the vacuum chamber VC. The number andsize of the gas bleed holes is related to the desired pressure withinthe enclosure and the desired flow rate. The source of the gas mightalso be a selfcontained source such as a self-enclosed gas bottle or anappropriate quantity of a porous material such as activated alumina,activated charcoal, or zeolite which will serve as a virtual leak in theenclosure. A material having a vapor pressure in the region of interestcould also be contained within the cavity as a source of gas or vapor.

The pilot cathode 1 requires only about 0.025 amperes to provide enoughpower (about 250 watts) at 10 kilovolts to heat the main cathode toemission temperature. Up until now, the emitters in electron beam gunswere customarily re- Sistance-heated by passing an electric currenttherethrough. Since large beam currents require a large heating currenton the order of 6S amperes, a strong electromagnetic field is createdwhich surrounds the emitter. The field electromagnetically bends theelectron beam away from the axis of the gun and therefore, is a sourceof beam distortion. The present invention utilizes a power electron beame.g. 250 watts or less to heat the emitter or main cathode for thehigh-powered '30,000-watt electron gun. This system has the advantage ofeliminating the electromagnetic field surrounding the emitter. The pilotcathode itself is virtually indestructible and will not require frequentreplacement. The main emitter may now fail only due to erosion orsputtering which is an uncontrollable process of vaporization ofextremely hot metals under vacuum conditions.

The present electron gun utilizes a temperature controlled emitter ormain cathode for purposes of varying beam current.

In practice, the magnitude of the main beam current is monitored by anappropriate circuit which sends a signal to a comparator circuit. Areference signal is also sent to the comparator circuit and thedifference in signals is then sent to the power supply of the pilot gunto raise or lower the power delivered thereto to increase or decreasethe pilot electron beam power. This, of course, raises or lowers thetemperature of the emitter material, thus producing only the requiredquantity of electrons, thereby increasing the average life of theemitter or main cathode.

Another advantage of the gun of this invention is that because the coldcathode technique requires only about 0.025 amperes, compared to the 65amperes used in prior guns, cables 31 and 33, which supply the pilot gunP, can be small and flexible so as to permit such cables to beconveniently coiled in and out of the chamber VC. For example, it ispossible for a 0.250-inch diameter cable to replace a 2.5-inch diametercable used with prior art guns, because that part of the cable bulk dueto the cross section required to carry high current and the heatingeffect of the high current is eliminated.

The gun G includes a pilot gun P operating at l0 kv. and a main gun Moperating at about 0.1-micron pressure at 60 kv. Electrically, the twoguns are independent, but they are connected through conductor 17,described above, to a common cable 31. Accordingly, cable 31 carries anoverall maximum potential of kv. The main cathode emitter 15 carries 60kv. and contacts the gas chamber C of pilot gun P and therefore, thereis a 601w. potential that might be applied through the gas to the wallsof the vacuum chamber VC. Thus, the gun could fire in the reversedirection by the 60 kv. if the 60 kv. can see a positive surface throughthe gas outlet or inlet holes.

The gun G is provided with several features for preventing theabove-described backfiring.

The first feature to eliminate backfiring through the gas input line 23is the aforementioned bubbler unit 25. This unit partially filled withdielectric fluid provides physical discontinuity between electricalground and the piping attached to the gun. This combination retards anybackfiring tendency through the inlet assage.

A second method of eliminating backfiring through the inlet line is tofeed the gas into the chamber through an insulated tube connected to agas bottle which is insulated from ground. Of course the two techniquesmay be employed simultaneously.

Another feature incorporated in the gun to minimize backfiring involvesthe use of the flange 9 on pilot anode 7. The flange 9 is at the samepotential as the pilot anode 7 and 60 kv. main cathode electrode 16.However, flange 9 is on the outside of the insulator material 2, puttingit closer to the chamber walls. This effectively moves the field stressoutside the gas region.

A third means of minimizing the tendency to backfire is to interpose astaggered hole bafi'le between the gas bleed holes and the gas plasma inthe enclosure such that there is no electrical line of sight" for thevoltage to see positive ground through the bleed holes. For example,three bleed holes might be provided in the top of the gun spaced apartsuch that the bleed holes appear at 0, 120, and 240. A ring bafflemounted below the area carrying the bleed holes would carryappropriately sized holes but spaced 60, 180, and 300 or any otherposition just so that the gas can find a path between the staggeredholes. Due to the fact that they never are in exact alignment electricallines of stress cannot see through them.

The lower baffle ring may either be an insulator or a conductor. When itis made of a conductive material a fourth means of backfiringsuppression is provided.

A fourth feature used to minimize backfiring involves incorporating acontrol ring outside the pilot anode through which the gas inlet andoutlet passages penetrate. The outer control ring contacts the Faradaydark space, and thus carries a high negative potential, which is relatedto pressure and applied voltage asshown in FIG. 6. As electrons approachthe high negative pbtential in the control ring, they are repelled whilepositive ions are neutralized. The control ring may or may not beconnected to the pilot anode through a high value of resistance.

The negative potential on the control ring is generated by takingadvantage of the various regions of electrical potential created in acold cathode discharge. It was found that the thickness of these regionsand the outside diameter of the Faraday dark space is inverselyinfluenced by ambient pressure. Thus, as pressure increases, thediameter of the region shrinks and thereby will move across a controlring positioned as shown. The ring acts like a slider of apotentiometer, and picks up a varying voltage across the dark space. Thevarying voltage picked up by the control ring is a function of voltage,pressure, current, cathode to ground surface distance, and position orgeometry of the dark space.

Because of these relationships it can be seen that variety of controlfunctions are possible both directly related to the invention of thepilot gun as well as other devices.

As shown in FIG. 6, there is a definite relationship between controlring voltage and beam current where pressure, electrode spacing andaccelerating voltage are held constant.

By applying a supplementary voltage to this ring the dark spacedimensions may also be varied to cause the beam current to vary. Whenthe negative voltage picked up by this ring is decreased by applying apositive potential or by grounding the rings self-potential through apotentiometer of approximately I megohm, the current will be reduced asthe resistance is reduced. A greater reduction in current may beobtained by choosing the parameters of pressure and spacing such that areduction in the negative potential on the control ring will cause thedark space to increase in depth until its lower edge makes contact witha grounded ring or surface. In this manner the control ring may be usedto modulate current flowing in the beam.

As the control ring potential also varies with changing cathode toground surface distance when voltage and pressure are maintainedconstant, the ring may be used as a transducer which produces an outputvoltage variation that is the analog of changing cathode to surfacedistance as might be required as a control signal for a surface contourfollower or a frictionless electronic cam follower.

Both the beam current and control ring voltage vary inversely aspressure varies when accelerating voltage and cathode to surfacedistance are held constant. Either or both parameters may be used tomeasure pressure over a range of at least 3 to more than 50 microns witha very high rate of response.

In the subject invention the negative potential picked up by the controlring with the discharge region is used as a self-biasing potential tohelp reduce baekfiring through the gas. This self-biasing could also beaided by the application of other voltage sources to the ring.

A further use of the self-biasing biasing potential picked up by thecontrol ring which again be supplemented by other voltage sources isshown in FIG. which is another embodiment of the invention.

FIG. 5 is similar to FIG. 4 in most respects but has another element 226added which functions to stabilize the focus of the cold cathode gun.Parts in FIG. 4 similar to the parts in FIG. I bear the same referencenumber, with the addition of one hundred thereto. Parts in FIG. 5 whichare similar to those in FIG. 4 bear the same reference number, increasedby one hundred.

A second embodiment of the gun is pictured in FIG. 4 generally shown atG. The pilot portion of gun G is shown at P. Such pilot gun has a pilotcathode member positioned in an insulator ring 102 which insulates thepilot cathode from the pilot anode I07. Attached to the pilot anode 107is a metal enclosure I04 which is at the same potential as the pilotanode 107. Between the pilot anode 107 and the enclosure 104 is acontrol ring 111 supported by and spaced from the anode by insulatorring 106 which also contains gas inlet hole [24 connected to gas inlettube 123. Positioned in the wall of the metal enclosure 104 opposite andin line with pilot cathode 101 is the main cathode emitter 115. The maincathode emitter is preferably a tungsten disc although any good electronemitting material such as tantalum or other metals coated with materialshaving a low work function could be used. Supporting the main cathodeemitter is main cathode electrode I16. Opposite and in line with maincathode electrode 116 is main anode 119 followed in line by main beamfocusing coil 12]. The entire assembly of the pilot gun is supported bya main gun insulator 114 which in turn is housed and supported byshielding cylinder 120 which is at ground or positive potential.

The two embodiments differ in that: FIG. I the pilot gun is assembledinto and electrically insulative housing with wire 17 acting to closethe electrical circuit between main cathode emitter I5 and pilot anode7. In FIG. 4 the pilot gun is assembled into an electrically conductivehousing in which the metal enclosure 104 is common to the main cathodeemitter IIS and pilot anode electrode 107.

Both systems function similarly although the electrical fielddistribution and shape of equipotential surfaces differ somewhat. Aninsulating sleeve may be inserted into the enclosure between the wallsof the metal enclosure and the gas space which would produce a fielddistribution in the FIG. 4 gun which would be more similar to the fielddistribution in the FIG. ll gun if desired.

It is possible to utilize a variable negative bias control in place ofor in conjunction with temperature variation of the main cathode emitterfor current control for the main gun. In such case, the main cathodeemitter 15 is insulated from the main cathode electrode and electricallyconnected to a variable bias supply which makes the emitter negativewith respect to the main cathode electrode.

A third embodiment FIG. 5 utilized a wire mesh cup or solid cup 226attached to the control ring 211, spaced with a gap or by an insulatingcylinder or insulating cone to prevent contact with metal enclosure 204.The cup at negative potential tends to remove positive ions from thebeam region while at the same time creating a pinching or focusingaction on the beam of electrons.

It is known that the focal length of the beam and its power density atthe focal point is related to the lens effect of the virtual anoderepresented by the equipotential surface at the lower extremity of theFaraday dark space. The area of the pilot cathode involved in thecreation of the beam determines the amount of current delivered by thegun. A low-current beam would make use of only the center portion of thepilot cathode, say an included angle of 20 as more current is drawn fromthe gun more of the cathode area glows and a greater included angle of,say 30 is involved. If boundary rays (straight lines) are drawn from theedges of the glowing area to intersect the axis of the gun the point ofintersection represents the focal point along the axis. It can,therefore, be seen that the focal point moves up as more current isdrawn and down as less current is drawn. When the angle is small, thefocal region is collimated to some degree and the beam is in approximatefocus over some distance, say 0.250" or greater. At larger angles, thefocal region approximates a point.

The shape of the lower edge of the dark space and the dimensions of thedark space are also a function of the changing involvement of the areaof the pilot cathode.

Without further refinements it can be said that the shape of cathodedetermines the range of focus for the range of current and pressureanticipated. Focus may further be accomplished by the use of permanentmagnet fields or by appropriately shaped electromagnetic fields astaught by the science of electron optics.

It can be seen, through anexperirnental glass enclosure that the darkspace grows thicker and comes closer to the cathode emitter 15 when thepressure approaches 5 microns as the reduced beam current tends tocollimate the beam or to move the focal point beyond the emittersurface. Under these conditions the beam current tends to its minimumand the voltage on the control electrode tends toward its maximum whichcould approximate 250 to 2,000 volts under certain conditions. On theother hand, when the pressure rises to approach microns, the dark spacethins and moves closer to the pilot cathode surface and a greaterportion of the pilot cathode is involved in the glow, thus, increasingthe beam current.

Under these conditions the voltage on the control ring may drop to or 50volts as the pressure approaches 20 microns and beam current approachesmaximum. The focal point moves higher and the beam at the emittersurface is divergent.

By connecting the cup 226 to control ring 2H1 the negative potentialfrom the control ring is applied to the sides and bottom of the cup. Thebottom of the cup is fitted with an aperture, the lips of which mayeither be the thickness of the cup material or increased by inserting atube in the aperture to increase its electrically effective length. Thisaperture is large enough, say 0.250" or larger, to pass the main body ofthe beam when the focal point is at the midpoint of the aperture butsmall enough to intercept a small portion of the fringe electrons fromthe beam. The charge on the cup will be the sum of two electrostaticcharges. If the focal point of the beam then moves higher the portion ofthe charge obtained from the control ring will decrease but at the sametime, the portion of the charge coming from the interception of thefringe electrons of a divergent beam will increase. The net increase inthe voltage on the cup and control ring will force the dark space tothicken (or move downward), thus forcing the focal point downward untilthe original equilibrium of the focal point in the aperture isreinstated.

This explanation could be made in terms of the fact that an increasednegative potential on the control cup would attract and neutralize morepositive ions, reduce the bombarding effect and secondary emission fromthe pilot cathode, and change the angle of area involvement on the faceof the pilot cathode but the sense of explanation would remain the same.

The above explanation describes the stabilization of the focus of thebeam to compensate for increases in pressure within the enclosure. Asimilar action takes place that (although less critical due tocollimating effects at lower pressures) stabilizes the focus whenpressure decreases.

As the pressure in the enclosure decreases the control ring potentialwill rise which would tend to decrease beam current and to decrease theangle of the pilot cathode area involved also the focal point would tendto move downward. Under these conditions the beam is of smaller diameterand does not tend to impinge fringe electrons upon the aperture edges tothe same degree that high-pressure defocalization does. However, therelatively great increase of voltage on the control cup will tend toremove more positive ions from the beam which will neutralize a portionof the negative potential on the control ring which will allow the darkspace to rise, increase the area of cathode involvement, increase beamcurrent and to move the focal point upward to reinstate focalequilibrium in the aperture.

At all times the electrical servosystem will drive the acceleratingpotential in the proper direction to keep the emission from the cathodeemitterat the preset value. The above stabilization is only an aid andthe working of the system is not wholly dependent upon it.

Having described the invention with reference to certain preferredembodiments, it should be understood that certain modifications may bemade to parts of the invention with respect to the arrangement thereofwithout departing from the spirit and scope of the invention.

What is claimed is: 1. An electron beam gun comprising: (A) a pilot gunhaving (i) An enclosure provided with a relatively low-pressureionizable gaseous medium providing the source of electrons from saidpilot gun; (ii) A pilot cathode positioned in one wall of saidenclosure, said pilot cathode having a concave surface to give aconvergent beam of electrons having a focal point beyond the Faradaydark space; (iii) A pilot anode positioned in said wall around andspaced from said pilot cathode the potential difference between saidpilot cathode and pilot anode providing the electron acceleratingvoltage for the electrons generated from said gaseous medium.

(18) A main cathode at the same potential as said pilot anode positionedin the wall opposite said pilot cathode and at a distance from saidpilot cathode beyond the Faraday dark space such that said main cathodeis bombarded by electrons from said pilot gun and thereby itself emitselectrons.

(C) A main anode spaced from said main cathode and having a highelectron accelerating voltage therebetween to accelerate the electronsemitted from said main cathode toward a workpiece.

2. An electron beam gun according to claim 1 wherein said pilot cathodehas a concave surface whose radius of curvature is approximately twicethe cathode radius.

3. An electron beam gun according to claim 1 including means for flowinggas from a source thereof into said enclosure to provide and maintainsaid low-pressure ionizable gaseous medium.

4. An electron beam gun according to claim 1 including a self-containedsource of gas in said enclosure.

5. An electron beam gun according to claim 3 and including a controlring positioned in said wall around said means for flowing gas into saidenclosure and in contact with the Faraday dark space when the gun is inoperation to thereby carry a high negative potential which repelselectrons.

6. An electron beam gun according to claim 4 having a control cup fittedwith an aperture at the bottom, in proximity to and in line with thecathode emitter for the purpose of aiding in the stabilization of thefocal region of the beam, is positioned in said enclosure.

7. An electron beam gun according to claim 1 wherein power is suppliedto said pilot gun through power cables having an outside diameter ofabout 0.250 and a conductor of as small as 400 circular mils crosssection.

1. An electron beam gun comprising: (A) a pilot gun having (i) Anenclosure provided with a relatively low-pressure ionizable gaseousmedium providing the source of electrons from said pilot gun; (ii) Apilot cathode positioned in one wall of said enclosure, said pilotcathode having a concave surface to give a convergent beam of electronshaving a focal point beyond the Faraday dark space; (iii) A pilot anodepositioned in said wall around and spaced from said pilot cathode thepotential difference between said pilot cathode and pilot anodeproviding the electron accelerating voltage for the electrons generatedfrom said gaseous medium. (B) A main cathode at the same potential assaid pilot anode positioned in the wall opposite said pilot cathode andat a distance from said pilot cathode beyond the Faraday dark space suchthat said main cathode is bombarded by electrons from said pilot gun andthereby itself emits electrons. (C) A main anode spaced from said maincathode and having a high electron accelerating voltage therebetween toaccelerate the electrons emitted from said main cathode toward aworkpiece.
 2. An Electron beam gun according to claim 1 wherein saidpilot cathode has a concave surface whose radius of curvature isapproximately twice the cathode radius.
 3. An electron beam gunaccording to claim 1 including means for flowing gas from a sourcethereof into said enclosure to provide and maintain said low-pressureionizable gaseous medium.
 4. An electron beam gun according to claim 1including a self-contained source of gas in said enclosure.
 5. Anelectron beam gun according to claim 3 and including a control ringpositioned in said wall around said means for flowing gas into saidenclosure and in contact with the Faraday dark space when the gun is inoperation to thereby carry a high negative potential which repelselectrons.
 6. An electron beam gun according to claim 4 having a controlcup fitted with an aperture at the bottom, in proximity to and in linewith the cathode emitter for the purpose of aiding in the stabilizationof the focal region of the beam, is positioned in said enclosure.
 7. Anelectron beam gun according to claim 1 wherein power is supplied to saidpilot gun through power cables having an outside diameter of about 0.250and a conductor of as small as 400 circular mils cross section.